1. Field of the Invention
This invention relates to methods and means for determining the presence of a ligand in a liquid medium based on the affinity of the ligand for a specific binding partner thereof. In particular, this invention relates to methods and means for use in specific binding assays which do not employ radioactive materials or modified enzymes as the labeling substance.
The desirability of a convenient, reliable, and nonhazardous means for detecting the presence of low concentrations of substances in liquids is self-evident. This is particularly true in the field of clinical chemistry where constituents of body fluids which may appear in concentrations as low as 10.sup.-11 molar are known to be of pathological significance. The difficulty of detecting such low concentrations is compounded in the field of clinical chemistry where sample size is usually quite limited.
Classically, substances have been detected in liquids based on a reaction scheme wherein the substance to be detected is a necessary reactant. The presence of unknown is indicated by the appearance of a reaction product or the disappearance of a known reactant. In certain instances, such an assay method may be quantitative, based on a measurement of either the rate of appearance of product or disappearance of reactant or measurement of the aggregate amount of product produced or reactant consumed in attaining equilibrium. Each assay reaction system is necessarily either limited to use in the detection of only a small group of substances or is non-specific.
2. Description of the Prior Art
The search for assay systems which are highly specific yet adaptable to the detection of a wide range of substances has evolved the radioimmunoassay. In this system a known amount of a radiolabeled form of the substance to be detected is allowed to compete with the unknown for a limited quantity of antibody specific for the unknown. The amount of the labeled form that becomes bound to antibody varies inversely with the level of unknown present. Inherent in the radioimmunoassay technique is the need to separate the labeled form of substance to be detected which becomes bound to antibody, the bound-phase, from that which does not become so bound, the free-phase. While various ways of accomplishing the required separation have been developed, as exemplified in U.S. Pat. Nos. 3,505,019; 3,555,143; 3,646,346; 3,720,760; and 3,793,445, all require at least one separate manipulative step, such as filtering, centrifuging, washing, or draining a column to insure efficient separation of the bound and free phases. Such separation is often accomplished by forming a system comprised of an insoluble portion containing the bound-phase and a liquid portion containing the free-phase such that the amount of radioactive label in either portion is a function of the extent of binding of the labeled material, and thus a function of the amount of ligand in the sample tested. The term "heterogeneous" as generally used by the scientific community and as applied herein, means those specific binding assays wherein a separation of the bound- and free-phases is accomplished. Such a separation is necessary to carry out a specific binding assay where the labeled material in the bound-phase is indistinguishable from that in the free-phase.
Because of the hazard and difficulty of handling radioactive materials, there have been many attempts to devise convenient specific binding assay systems which are as sensitive and rapid as radioimmunoassays but which utlize features other than radioactivity as the means for monitoring the binding reaction. As will be discussed more fully hereinafter, materials which have been utilized as the labeling substance in place of radioactive atoms or molecules include such diverse materials as enzymes, fluorescent molecules, and bacteriophages.
Exemplary of methods which have been developed using an enzyme as the labeling substance are those described in U.S. Pat. Nos. 3,654,090; 3,791,932; 3,839,153; 3,850,752; and 3,879,262 and in the Journal of Immunological Methods 1:247(1972) and the Journal of Immunology 109:129(1972). In each of the described methods an enzyme is chemically coupled to either the ligand to be detected or a binding partner thereof and an appropriate heterogeneous specific binding reaction scheme is constructed whereby after incubation with a sample, the amount of enzymatic activity associated with either the insoluble portion or the liquid portion is a function of the amount of ligand in the sample. The problems associated with the synthesis and characterization of the enzyme-conjugates are serious short comings of this approach.
Of interest is the enzyme-tagged immunoassay described in U.S. Pat. No. 3,817,837. This method does not require the use of a partitioned (i.e. insoluble portion/liquid portion) specific binding reaction system and the separation procedure necessitated thereby since the enzyme-tagged ligand is designed such that upon reaction with the binding partner of the ligand, enzymatic activity is inhibited. Thus, the ratio of bound tagged material to that in free form can be determined by monitoring changes in enzymatic activity. Nonetheless, this method suffers from the difficulty of preparing well-characterized enzyme-tagged conjugates and of finding enzymes that will fit the basic design of the system.
British Pat. No. 1,392,403 and French Pat. No. 2,201,299, which patents correspond to U.S. Pat. No. 3,880,934 describe a specific binding assay which utilizes a non-active precursor of a spectrophotometrically-active substance as the labeling substance. After incubation of the sample with the specific binding reaction system, the insoluble and liquid portions are separated and the amount of labeling substance present in the liquid portion, which is a function of the amount of ligand to be detected in the sample, is determined by carrying out reaction steps that transform the inactive labeling substance into a chromogen or fluorometrically active material which is then measured by conventional means.
Other specific binding assay methods employing different types of labeling substances are disclosed in: U.S. Pat. No. 3,850,578 which discloses the use of electron spin resonance as a labeling means; U.S. Pat. No. 3,901,654 which discloses the use of fluorescense quenching and enhancement as a labeling means; and Report No. PB-224,875 of the National Technical Information Service (NTIS) of the United States Department of Commerce (1973) which describes an unsuccessful attempt to use hemin chloride as a labeling substance in a heterogeneous assay system monitored by a chemiluminescence reaction. Nature 219:186(1968) describes in great detail certain radioimmunoassay procedures and makes a passing reference of a very general nature to the possible use of coenzymes and viruses in place of radioisotopes as labeling substances. However, the author provides no enlightenment as to how to carry out an assay using such alternative labeling substances, or in fact as to whether such an assay would be operable. For further background, reference may be had to Principles of Competitive Protein-Binding Assays, ed. Odell and Daughaday (J. B. Lippincott Co., Philadelphia, 1972) which discusses in breadth the various known assay schemes and the different materials and features that have been used as labels for specific binding assays.
Even though many new types of specific binding assays have been suggested and investigated, the radioimmunoassay and the various enzyme-tagged immunoassays remain the most widely used and improved. However, both types of systems have obvious shortcomings, the radioimmunoassay in its use of radioactive material which is hazardous and requires careful handling and the enzyme-tagged immunoassays in the difficulty of preparing useful enzyme-tagged conjugates.
It is therefore an object of the present invention to provide a novel method and means for detecting a ligand in a liquid which do not employ inconvenient radioactive materials or modified enzymes as the labeling substance.
Further, it is an object of the present invention to provide a heterogeneous specific binding assay method and means which are more versatile and convenient than those of the prior art.
Another object of the present invention is to provide a heterogeneous specific binding assay method and means which employ a labeling substance which is capable of being coupled to the ligand or to a specific binding partner thereof more conveniently than can the enzyme of the prior art method.
It is also an object of the present invention to provide a heterogeneous specific binding assay method and means which employ a conjugate comprising a labeling substance which is more conveniently detectable using a wide variety of sensitive monitoring reaction systems than is the enzyme in the prior art method.